Integrated circuit devices are typically subject to rigorous testing before they are sold or put to their intended use. In particular, each integrated circuit device is tested to determine whether or not certain specifications for that type of device, as determined and set by the manufacturer, are met.
Complete testing of an integrated circuit device comprises both functional testing as well as application-specific testing. Functional testing relates to whether or not the hardware found within the integrated circuit device is free from defects and meets manufacturers' specifications. For example, if the integrated circuit device is or comprises a memory component such as a memory module or a memory device, functional testing relates to whether or not a digital value written to a cell of memory will later be retrieved without error, regardless of how the memory module or memory device is implemented.
Functional testing also relates to whether or not certain critical operating characteristics of the integrated circuit device fall within the allowable range of values. These critical operating characteristics include such characteristics as power consumption, standby current, leakage current, voltage levels, and access time. The allowable range may equally be set by the manufacturer of the device or by the corresponding appropriate standards.
Functional testing is oriented toward discovering whether or not the integrated circuit device under test is likely to fail during its intended use or application. It involves testing integrated circuit devices to verify how they execute a set of functions during testing procedures that are specifically designed for this purpose.
Complete testing of an integrated circuit device also involves application-specific testing. During application-specific testing, integrated circuit devices are subject to testing of their system behavior in order to detect their behavioral failures. Behavioral failure is a type of failure that occurs when an integrated circuit device is operated within an actual application system. For example, it may be a failure that occurs as a result of a specific command or access sequence to a memory device or memory module found in normal PC operations.
It is not necessarily the case that functional testing will detect behavioral failures because, during that type of testing, the operation of the integrated circuit device under test is not necessarily indicative of how the device will behave during its intended application. Accordingly, complete and comprehensive testing of an integrated circuit device requires application-specific testing in addition to functional testing. Functional testing alone is not sufficient.
Though they may have different objectives, both functional and application-specific testing of integrated circuit devices involve the use of test vector patterns. In particular, in either case, test vector patterns are generated by an appropriate test vector generator and then transmitted to the device under test across a communication channel. Ideally, the test vector patterns are transmitted at a rate that is equivalent to the full processing speed of the integrated circuit device because that is the speed at which the device would ideally operate under normal applications.
The data processing speeds at which integrated circuit devices process data are ever increasing. Current processing speeds typically exceed 1 GHz. Future processing speeds might reach 20 GHz or higher. In the context of testing integrated circuit devices, the increasing processing speeds of integrated circuit devices correlates to greater demands on the transmission rates of the test vector patterns. Only where the transmission rate of the test vector can match the full processing speed of the device will the most useful and rigorous testing of the device take place.
However, test vector patterns must be transmitted to the device under test over a communication channel, and all physical communication channels impose passband limitations on the electrical signals passing over them. Common communication channels used in the testing of integrated circuit devices include coaxial cables and Printed Circuit Board (PCB) layout traces. Other types of communication channels are known.
One aspect of the communication channel's passband is a high-frequency cutoff characteristic with respect to transmitted signals. In other words, all frequencies below a critical frequency will transmit with little or no attenuation of signal strength or distortion of edges. However, all frequencies above that critical frequency will only transmit with attenuation of signal strength. As a result, a mixed frequency electrical signal potentially may transmit with very significant signal distortion or attenuation, depending on how much of the signal's frequency content falls above the critical frequency.
An additional constraint on a maximum frequency of test vector waveforms transmitting across communication channels is degradation of waveform integrity on account of reflections. Electrical waveforms transmitted across communication channels have the potential to reflect at the channel's terminal points. Those reflections superimpose onto the original waveform causing distortion in the form of degradation of signal integrity. Mitigating the effect of waveform reflection involves limiting the period of a transmitted waveform to above a certain threshold value. In terms of frequency, this constraint manifests as a limit on a maximum frequency for transmitted waveforms at which no practically significant distortion occurs, in line with the constraint imposed by the passband characteristic of the communication channel.
Depending on either or both of the passband or the reflection characteristic of the communication channel used in the device test, the desired test vector patterns to be used in the test often will not transmit across the communication channel without significant distortion. If the desired test vector has significant frequency content falling outside the passband of the communication channel, then the resulting distortion may render the test vector unusable for testing purposes. It is possible that signal distortions of this kind might not exhibit itself within real application systems, in which signals typically travel much shorter distances and in which signal connections are optimally terminated.
Test vector patterns, when transmitted over a communication channel to the device under test, are in the form of time-varying electrical signals. At the input to the device under test, the transmitted electrical signal is converted into its digital representation through the process of sampling. The well-known Nyquist Theorem provides a fundamental relationship between sample rate and the maximum allowable frequency of the electrical signal undergoing sampling. According to the theorem, in order to avoid aliasing effects, the electrical signal must be sampled at a rate that is equal to at least twice the maximum frequency of the signal undergoing sampling.
According to current processes, sampling of electrical signals often involves the use of a sampling clock, typically a square-wave pulse train. In one form, samples of electrical signals are taken at every rising or every falling edge of the sampling clock. In this manner, the sampler will take one sample of the signal every period of the sampling clock. However, in another form, samples are taken at every rising edge, as well as every falling edge, of the sampling clock. In this manner, the sampler will take two samples for every period of the sampling clock. It is therefore possible, using this second form of sampling, to achieve twice the sampling rate for a given clock frequency, or else half the clock frequency for a given sampling rate.
Thus, accepted communication theory establishes a direct relationship between the processing speed of the integrated circuit device and the passband of the communication channel for ideal testing. Under ideal conditions, the integrated circuit device samples at a rate equal to its processing speed. That fixes the transmission rate of the test vector pattern across the communication channel at an equal rate, which in turn fixes the required passband of the communication channel. The basic relationship is that, in order for the communication channel to transmit every possible test vector pattern without distortion under these conditions, it must have a passband that is not less than half of the full processing speed of the integrated circuit device. For example, if the device processes at 2 Gb/s, then the channel should have a passband of at least 1 GHz.